The present invention relates to a power transmission belt for transmitting torque between two pulleys having a driving surface of generally V-shaped cross-section. The power transmission belt is particularly fit for use in a continuously variable transmission for an automotive vehicle.
There is known a power transmission belt as shown in U.S. Pat. No. 4,313,730 issued to Cole, Jr. et al on Feb. 2, 1982. This known power transmission belt comprises an assembly of a single chain constructed of a plurality of sets of interleaved links joined to its next adjacent set by a pin, and load blocks surrounding the chain and located between the next adjacent pins. Each of the load blocks fit over one set of links to be carried thereby. The load blocks are generally contoured at their edges to fit into the V of a pulley. When the load blocks are close together, the front and rear surface of at least some of the load blocks are tapered to permit articulation of the assembly.
This power transmission belt extends between two pulleys when, in use, a torque is to be transmitted between them. The chain of the belt articulates in a polygonal manner when it winds around each pulley. Thus, the trace drawn by each of the pins of the chain is not a part of a circle about the pulley rotation axis but a part of the polygon. In operation, the rotation of the driver one of the pulleys causes the load blocks to come into engagement with the driving surface of the pulley one after another. Let us now consider one pin joining two adjacent sets of interleaved links, the leading set carrying a load block which has come into engagement with the driving surface of the pulley, while the trailing set carrying a load block which is about to engage the driving surface of the pulley. Assuming that the running radius of the power transmission belt is R, the position assumed by this pin is expressed by the running radius R from the pulley rotation axis. After the pulley has rotated through an angle .theta. to let this pin advance half the pitch P (the pitch P=the distance between the adjacent two link pins), the pin assumes a position which is displaced radially outwardly of the running radius R by a radial distance .DELTA.L. This position is expressed by R/cos .theta.. Thus, the radial displacement .DELTA.L after the advance by half the pitch is expressed by the following equation: EQU L=R/cos .theta.-R=R(1/cos .theta.-1).
The angle .theta. is expressed by .theta.=tan.sup.-1 (P/2 R). Since, in the above mentioned manner, the pins of the chain moves radially outwardly away from the pulley rotation axis and then radially inwardly toward the pulley rotation as it rotates around the pulley, the chain is forced to vibrate in sinusoidal manner with an amplitude of .DELTA.L.
This vibration of the chain causes the load blocks to impinge upon the driving surface upon coming into engagement with the pulley, producing a periodical impinging sound. Since the overall length of the power transmission belt is invariable, the vibration causes the belt to elongate if the amplitude .DELTA.L is relatively great. Owing to this vibration, the pulleys are forced to vary the distance therebetween, causing the input and output shafts to vibrate. The periodical impinging sound combines with the vibration of the shafts within a case surrounding the pulleys, causing the case to emit a relatively large noise outwardly.
Since the pitch of the chain is constant, the noise with the frequency corresponding to the first order of the pitch becomes considerably large.
The relationship between the pitch and the noise is such that the longer the pitch, the larger the amplitude becomes so that sound producing energy increases accordingly.
For example, assuming R=50 mm, P=8 mm, .DELTA.L can be given as,
.DELTA.L=0.16 mm. PA1 .DELTA.L=0.04 mm.
Assuming P=4 mm, .DELTA.L can be give as,
Therefore, the amplitude .DELTA.L increases in proportion to the second power of the pitch P.
From the preceding description, it will be noted that in order to suppress the noise, the pitch of the chain should be shortened to decrease the radial component of the motion of the belt. However, the present chain design demands the pin diameter and the chain width to decrease as the pitch of the chain decreases. Thus, reducing the length of the pitch results in a reduction in the structural strength of the chain. Considering its use in an automotive vehicle having a 2000 cc internal combustion engine, the pitch of the chain can not be made shorter than a range from 9.5 mm to 8 mm.